Bergmann’s rule is a widely studied zoogeographic, ecogeographic trend of increased body size in cooler climates. It, along with others (e.g. Allen’s rule), are empirical generalizations of the observed correlation between variation in the environment and morphology. Although originally formulated to explain interspecific variation within broadly endothermic vertebrate genera, Bergmann’s rule has been subsequently studied in narrower scales (i.e. intraspecific variation) as well as broader scales (i.e. interspecific variation within larger taxonomic scales, such as a family). Empirical support has been mixed among clades, and it varies among taxonomic levels. Taking advantage of greatly improved phylogenetic resolution among rodents, we tested Bergmann’s rule using 19 bioclimatic variables, and body mass data, for 1,315 species on a recent supermatrix phylogeny. Our basic question is, within the taxonomic scale of order Rodentia, does interspecific body mass variation occur in fashion consistent with Bergmann’s rule (i.e. larger species occur in colder climates and smaller species occur in warmer climates)? We investigated this correlation both directly, and after correcting for phylogenetic relationships using phylogenetic generalized least squares (PGLS) analysis based on the aforementioned supermatrix phylogeny.
We did not find the predicted negative relationship between body mass and temperature. Instead, the PGLS analysis indicated that precipitation variables (especially variables associated with primary productivity) had a positive correlation with body mass, suggesting that rodent species tend to be larger in more productive regions. Multivariate regression between body mass and overall climate (based on 19 bioclimatic variables) found a significant relationship, that was robust to phylogenetic correction. Bergmann’s rule was not detected in smaller and surface-dwelling rodents (despite their greater exposure to external climate), any more than in larger and subterranean rodents. Thus, our results indicate that Bergmann’s rule does not operate in the broad taxonomic scale of the order Rodentia—temperature does not seem to influence body size in rodents at this level, in contrast to many studies in other mammals, conducted at scales that are more limited. We suggest that food availability, and not heat conservation, is the more important mechanism driving body size variation across rodent species at the order level.